Recent increases in bovine brucellosis have raised serious doubts as to whether the disease can be eradicated using present techniques. While the conventional use of live strain 19 Brucella abortus vaccine has dramatically decreased the disease incidence over the past several decades, it now appears that either improved vaccines or improved diagnostic tests may be required to complete the eradication process.
We now know that there are two types of immunity involved in disease resistance. The first type of protection-humoral immunity-is afforded by antibodies in the circulation and body secretions. A second type of immunity is mediated by specialized whole blood cells (lymphocytes) and is thus deemed cell-mediated immunity (CMI). Recent studies have shown that cell-mediated rather than humoral immunity is involved in brucellosis resistance.
Many of the problems regarding brucellosis eradication arise from the fact that persistant antibody levels from the standard calfhood strain 19 vaccine interfere with laboratory techniques utilized in diagnosing naturally infected animals. Standard tube and plate agglutination tests record increased antibody titers which make it very undesirable to immunize adult cattle in infected herds, since immunized animals cannot be readily distinguished from infected animals. Also, the protection resulting from strain 19 vaccination is only 65-75% effective.
Strain 19 which stimulates both humoral immunity and CMI is especially undesirable for use in calves in certified-free areas or as an aid in the control of brucellosis in cows in heavily infected herds, due to false-positive titers.
Currently employed serological tests record the presence of humoral antibodies. It would be desirable therefore to add to the armamentarium of the veterinarian a vaccine which would stimulate the cell-mediated arm of the immune response without also stimulating circulating antibodies which react in the standard serologic tests employed at present. By stimulating only CMI, which is responsible for the clearance of intracellular parasites such as Brucella, present serological tests would indicate exposure or infection with naturally occurring B. abortus and not strain 19 or other vaccinal strains. Such a vaccine could also be used in adult cattle to control the disease in problem herds.
Vaccines comprising whole, killed Brucella abortus strain 45/20 (BA 45/20) cells, a rough strain, in inert oils such as paraffin oil or mineral oil have been used outside of the United States. These vaccines are not completely satisfactory for a number of reasons, principally because they require two initial doses followed by an annual booster injection (McDiarmid, Ann Inst Pasteur 102:792-800, and Hendricks et al, U.S. Department of Agriculture, Animal Health Division Staff Report, 1966). Additionally, adverse reactions at the site of innoculation commonly occur with some commercial water-in-oil vaccines.
Vaccines based on live cells, attenuated cells, killed cells and cellular antigens are currently used or have been reported. None are completely satisfactory. There is a genuine need for an improved vaccine which can be safely and reliably employed without the above-cited disadvantages.